The present invention relates generally to an improved system for the delivery of physiologically active agents and, more particularly, to an improved delivery system for the timed release of physiologically active agents.
In recent years numerous devices have been devised which utilize osmotic flow to assist in the delivery of physiologically active agents. For example, both U.S. Pat. No. 4,265,874 to Bosen et al and U.S. Pat. No. 4,298,003 to Theeuwes et al disclose methods and devices for the delivery of a drug where, as a result of osmotic flow, fluid passes through a semipermeable membrane and forces an insoluble drug or a solution of a soluble drug out of the device through an enlarged opening or passageway. The membrane of these devices allows the flux of water only, not the drug. The drug is forced out, under the influence of osmosis, through the enlarged opening or passageway which is separately drilled in the devices and whose size is orders of magnitude larger than the pores of the membrane. U.S. Pat. No. 3,832,458 reveals a device in which a silicon polymer wall is utilized to vary permeability to an internal active agent. The permeability is adjusted by fabricating the wall with varying amounts of N-vinyl-pyrrolidone. While this device represents an improved delivery technique, it has a significant disadvantage in that it represents a "first order" delivery system in which the driving force of drug delivered to the outside is the result of the internal concentration of drug alone. Thus the drug will be delivered at an initial rapid rate followed by a significantly lower rate until the active agent is expended. U.S. Pat. No. 4,309,996 by Theeuwes discloses a somewhat different mechanism for delivery of drugs whereby a separate compartment filled by a net osmotic inflow is utilized to expand against a flexible internal partition which forces active agent out of a second compartment through a microporous structure thus attempting to approximate a steady delivery rate.
In summary, while the prior art devices have resulted in improved delivery techniques, they are either somewhat complex, adding to the cost of the devices, or unable to control the precise drug delivery rate. It is therefore advantageous to provide a system which is flexible in that there are numerous variables which can be modified to control the delivery rate of the drug.